1. Field of the Invention
The present invention relates to a photomask, to a method of manufacturing the same, and to a method of using the photomask to measure the aberration of an image produced by a lens. In particular, the present invention relates to a method of measuring the aberration produced by a lens of an exposure apparatus used in the manufacturing of semiconductor devices or the like.
2. Description of the Related Art
In fabricating semiconductor devices or liquid crystal displays (LCDs), an exposure apparatus irradiates a substrate with light directed through a photomask, bearing a predetermined pattern, to thereby form a desired pattern on the substrate. More specifically, the pattern defined by the photomask is formed on the substrate on the same scale or at a reduced scale in the case of a reduction projection exposure apparatus. To this end, the exposure apparatus also includes an optical system having a lens. However, due to limitations on the design and manufacturing of the lens, the lens of the optical system produces an aberration, i.e., errors in the image formed by the optical system on the substrate. For example, the projection lens of a reduction projection exposure apparatus can produce errors such as a spherical aberration, astigmatism, or coma aberration. In order to minimize the effects of the aberration produced by a lens of an optical system, it is important to precisely measure what will be hereinafter referred to as the aberration of the lens.
In general, any specific feature of the photomask image to be transferred to a substrate is transferred through the entire pupil of the lens during an exposure process. Accordingly, it is difficult to use such an exposure process to measure the lens characteristics of only a specific region on the plane of the pupil of the lens. In order to analyze the pupil of the lens, it is necessary to form an image by passing optical beams through specific locations on the pupil of the lens. In other words, information on the pupil of the lens can be attained by separating the plane of the pupil into small regions and creating an image by transferring light features through the small regions.
In a conventional method of measuring the aberration of a lens, an image is formed on a substrate by passing an optical beam through a photomask, and then through a specific region of the plane of the pupil of the lens via apertures disposed under the photomask as formed separately therefrom. The apertures are used for determining the aberration characteristics of a specific region of the pupil by limiting the angle at which the optical beam is incident on the lens. That is, the apertures cause the beam, which has passed through a transmissive portion of the photomask having a predetermined shape, for example, that of a bar, to pass through the lens at a specific angle.
However, the prior art apparatus for measuring the aberration of a lens is complex as it requires separate apertures installed on or under the photomask. In addition, the apertures have to be mechanically moved to precisely align the photomask and the apertures. The alignment process requires a large amount of time and compromises the precision of the results of measuring the aberration of the lens.